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A vaccine that protects women against cervical cancer also appears to protect them against throat cancers caused by oral sex, and presumably would protect men as well, according to a study released Thursday.

Rates of this throat cancer have soared in the past 30 years, particularly among heterosexual middle-aged men. About 70 percent of oropharyngeal cancers are now caused by sexually transmitted viruses, up from 16 percent in the 1980s. The epidemic made headlines last month when the actor Michael Douglas told a British newspaper that his throat cancer had come from performing oral sex.

Oncologists have assumed that the human papillomavirus vaccine, which is used to prevent cervical cancer, would also prevent this other type of cancer, but this was the first study to provide evidence.

“This is a very nice paper,” said Dr. Marshall R. Posner, medical director for head and neck cancer at Mount Sinai Medical Center in New York, who was not involved in the study. “We expected this — that’s why we want everyone to vaccinate both boys and girls. But there’s been no proof.”

The study, supported by the National Cancer Institute, found that Cervarix, made by GlaxoSmithKline, provided 93 percent protection against infection with the two types of human papillomavirus that cause most of the cancers.

“We were surprised at how big the effect was,” said Dr. Rolando Herrero, head of prevention for the World Health Organization’s International Agency for Research on Cancer, and the study’s lead author. “It’s a very powerful vaccine.”

The study was done with 5,840 women in Costa Rica who were ages 18 to 25 and sexually active when it began. Four years after being vaccinated, each gave a mouthwash gargle sample that picked up cells from deep in the throat. Only one woman who had received the vaccine was infected with the viruses HPV 16 or HPV 18, the cancer-causing types; 15 women who had gotten a placebo vaccine were infected.

Dr. Herrero explained some of the study’s limitations: when it began, it was concerned only with cervical cancer, so no men were enrolled. The women were initially tested to make sure they had no cervical infections, but were not tested for throat or anal infections. They gave oral samples only once, so it was not possible to say how many had persistent infections; most people clear HPV infections on their own, so only a tiny fraction lead to cancer. Four years is not long enough to know how many cancers would develop — but finding out for sure would require waiting 20 years or more, and ethical guidelines require that all women in the trial get regular examinations and that any suspicious lesions be destroyed before they turn cancerous. Also, only Cervarix, and not Merck’s similar Gardasil vaccine, was tested.

However, Dr. Herrero said, men would “probably” get the same protection as the women did, because the vaccine produces identical antibody levels in both sexes.

Dr. Posner said the large discrepancy in infection rates between those who got the vaccine and those who got placebo suggested that the data was “very reliable” even though the infections were detected far too early to produce cancers.

“What we don’t know,” he said, “is how long-term the protection is, or if re-vaccination is necessary.”

While cancers caused by smoking or drinking usually occur in the mouth, those caused by oral sex usually occur at the base of the tongue or deep in the folds of tonsillar tissue, and are hard to detect. They are more common among heterosexual men than among women or gay men; experts believe that is because vaginal fluid contains more virus than the surface of the penis.

Dr. Eric J. Moore, a Mayo Clinic surgeon specializing in these cancers, said the study was “very encouraging.”

“But remember,” he added. “It only works if you’re vaccinated prior to contracting the infection. Once you’re 40 and have had multiple sexual partners, it’s not going to help.”